1. Field of the Invention
This invention relates to a conductive composition for use in the formation of a printed circuit and a method for manufacturing a circuit using the conductive composition, more particularly, this invention relates to a conductive composition and a method for manufacturing a printed circuit substrate using the conductive composition which is therefore solderable.
2. Description of the Prior Art
Heretofore, a circuit substrate which can be directly soldered is obtained by etching a copper foil bonded to an insulating substrate or by printing and sintering paste which contains conductive particles of silver, platinum or palladium and fritted glass on an insulating substrate.
However, soldering a copper foil has disadvantages which tend to complicate the process steps, because etching is required and the copper foil is wasted by the etching process. Soldering a sintered paste of silver, platinum or palladium on an insulating substrate has drawbacks because the insulating substrate on which the circuit is formed is an expensive ceramic which can withstand the high sintering temperatures of 600.degree.-900.degree. C. Moreover, the ceramic substrates are mechanically easily damaged during the sintering process. Furthermore, the sintering equipment is sizeable and expensive.
The conventional technology has produced a conductive composition to form a circuit which can be soldered with less expensive material and in simpler steps. Since paste-like conductive composition that contain synthetic resin which can be cured at relatively low temperature is heretofore used as a binder to form a printed circuit, the production of a composition which can be soldered has been attempted on the basis of the conductive composition. As an example, conductive paste in which silver powder is dispersed in epoxy resin or conductive paste in which copper powder is dispersed in phenol resin is printed and baked on a synthetic resin laminated board has been conventionally used.
However, since the above-described conventional conductive composition is covered on the surface of a coating film with the resin so that the ratio of conductive particles to be exposed on the surface is small and the area to be contacted with the solder is small, the conventional conductive composition has a drawback that the bonding strength of the coating film to the solder is weak. Further, the quantity of the conductive particles is increased to increase the bonding strength to the solder, but since the quantity of the resin is decreased, there arises new drawback that the bonding strength between the insulating substrate and the coating film is deteriorated. Moreover, a low melting point solder exhibits worse solderability and is not practical. When silver powder is used as conductive particles, the silver powder is diffused in the solder, decreasing bondability of the soldered silver to the substrate (so-called "silver diffusion" occurrs). Since the silver is expensive, the material costs are increased also. When copper powder is used as the conductive particles, the surfaces of the copper powder particles are oxidized thereby decreasing solderability.